Using a few trials work out what angle you have to hold the 4 minute one at for it to time exactly 2 minutes. Also work out some angles for other times for when some twit goes "OK smart alec, now do N minutes".
Then time 7 minutes with the 7 minute timer and 2 minutes with the 4 minute timer at the appropriate angle.

Using a few trials work out what angle you have to hold the 4 minute one at for it to time exactly 2 minutes. <obvious after this, so skipped>

Nice approach. But, I fail to understand how you measure "exactly 2 minutes" !! HOW ?! If you have a watch or something to measure "exactly" two minutes, then why go through all this at all ?? Your 7 mins hourglass can't help you too because 2/7 is not something you can "exactly" measure with naked eyes.
Kindly explain your approach "more" clearly. .... To be specific, please explain how you measure "exactly" 2 mins after a few trials ?!

@xpi0t0s
In questions like these you assume that your hourglasses are correct, else by your approach, how do you know your stopwatch is correct too? you assume it to be!
Or consider it that hourglasses were checked with a watch by a 3rd person and given to you...
Hence you cannot have anything else.....